AND DEVELOPMENT OF PYROSOMA. 219 
end of the saccular rudiment of the alimentary tract (figs. 15 and 16), and seems to be the 
means of connecting the end of that sac with the external tunic. 
After a time, however, a clear space appears around the apex of the sac, and separates 
this connecting mass from the rest, which now, consequently, appears as a broad zone (a) 
surrounding the sac below its apex, but above the uppermost of the branchial stigmata 
(fig. 19). This zone remains as a broad, thickish girdle of indifferent tissue, closely con- 
nected with the outer tunic externally and in front, and with the generative blastema 
behind, in buds of ^jth— , ^-th of an inch long (figs. 21 and 22) ; but in larger zooids its 
tissue has undergone a great change, and it has become a transparent mass, through 
which ramified corpuscles, like connective-tissue corpuscles, appear scattered (fig. 23). In 
this condition it is exactly analogous to the structure termed elseoblast in the Sa/pcc by 
Krohn. Its bulk is now equal to a fifth or a sixth of that of the entire bud, but in subse- 
quent stages (figs. 24, 25) it diminishes both absolutely and relatively in size and eventu- 
ally it disappears. 
In buds - 5 Vh of an inch in diameter, the generative blastema remains in its primitive 
condition, except that it and the ovisac it contains, have increased in size. Its anterior 
pointed end is closely juxtaposed to the endostylic cone. In the zooid represented in 
fig. 23, which measured - 3 Vth of an inch in length, the generative blastema has become 
divided into two parts, the smaller of which remains in close apposition to the endo- 
stylic cone, while the larger, retaining its connexion with the posterior and upper wall 
of the mid-atrium, becomes widely separated from the other. The interval between the 
two is occupied by the elseoblast. Even before the separation has taken place, the larger 
portion has become distinctly differentiated into two parts, the ovisac, on the left, sepa- 
rating itself from a rounded mass of indifferent tissue, on the right. This last is the 
rudiment of the testis. Erom rounded, it becomes pyriform, the narrowed neck of the 
pear remaining in connexion with the atrial wall, and eventually becoming metamorphosed 
into the vas deferens, while the broad end increases in size, and is directed more forwards 
well as upwards 
In a bud tUu of an inch long (fig. 24) the testis 
th of an inch in len 
while its broad end is above ^oth of an inch thick. The apex of the vas deferens already 
pushes a little eminence of the atrial tunic before it. 
In a young ascidiozooid, somewhat more advanced than that represented in fig 25, 
the vas deferens is ^th of an inch in length, and is of nearly even diameter throughout 
except at its upper end, where it is slightly dilated and plainly hollow. It is connected 
with the posterior part of the terminal enlargement, which is nearly too^ of an inch 
thick, and is divided into three short lobes, each about jh>^ ° f an mch "?*• Llke „ 
the previously existing pyriform enlargement, these rudimentary caeca are solid masses of 
indifferent tissue. Traces of a distinct membrana propria are discernible around each 
In still larger ascidiozooids the number of caeca increases, and the whole organ 
im. ^ „„*„. ^ & 
becomes larger, until it assumes its adult form ; and it is only when nearlj 
dition,that spermatozoa are visible in the vas deferens and the adjacent parts of the caeca. 
The development of the ovisac will be described below. At first both the testis and 
2g2 
